Jean Picard

Jean Picard, who lived from 1620 to 1682, was a French priest, astronomer and pioneer in geodesy, born in La Flèche, where he studied at the Collège royal Henri le Grand. He is best known for accurately measuring the size of the Earth: in a survey of 1669 and 1670 he measured one degree of latitude along the Paris Meridian by triangulation from Paris to Sourdon near Amiens, finding 110.46 km per degree, a figure Isaac Newton later used in developing his theory of universal gravitation, and a pyramid at Juvisy-sur-Orge commemorates the work. He was the first to attach a telescope with crosswires to a quadrant and one of the first to use a micrometer screw, which made his measurements 24 times as accurate as those of Tycho Brahe. In 1670 and 1671 he travelled to the site of Tycho's observatory at Uraniborg to fix its longitude, observed the aberration of light there and discovered mercurial phosphorescence from the faint glow of a barometer, and he developed the standard method of measuring right ascension using the pendulum clock of Christiaan Huygens. He corresponded with Newton, Huygens, Ole Rømer and his competitor Giovanni Cassini.